<img src="https://images.unsplash.com/photo-1507525428034-b723cf961d3e?w=753&q=80" width="753" height="500">
  <img src="https://images.unsplash.com/photo-1519046904884-53103b34b206?w=753&q=80" width="753" height="500">
  <img data-src="https://images.unsplash.com/photo-1495954484750-af469f2f9be5?w=753&q=80" width="753" height="500" lazy>
  <img data-src="https://images.unsplash.com/photo-1587502536900-baf0c55a3f74?w=753&q=80" width="753" height="500" lazy>
  <img data-src="https://images.unsplash.com/photo-1510414842594-a61c69b5ae57?w=753&q=80" width="753" height="500" lazy>
  <img data-src="https://images.unsplash.com/photo-1471922694854-ff1b63b20054?w=753&q=80" width="753" height="500" lazy>
  <img data-src="https://images.unsplash.com/photo-1473186578172-c141e6798cf4?w=753&q=80" width="753" height="500" lazy>
  <img data-src="https://images.unsplash.com/photo-1487349384428-12b47aca925e?w=753&q=80" width="753" height="500" lazy>
  <img data-src="https://images.unsplash.com/photo-1527437934671-61474b530017?w=753&q=80" width="753" height="500" lazy>
  <img data-src="https://images.unsplash.com/photo-1473496169904-658ba7c44d8a?w=753&q=80" width="753" height="500" lazy><img src="https://images.unsplash.com/photo-1507525428034-b723cf961d3e?w=753&q=80" width="753" height="500">
  <img src="https://images.unsplash.com/photo-1519046904884-53103b34b206?w=753&q=80" width="753" height="500">
  <img data-src="https://images.unsplash.com/photo-1495954484750-af469f2f9be5?w=753&q=80" width="753" height="500" lazy>
  <img data-src="https://images.unsplash.com/photo-1587502536900-baf0c55a3f74?w=753&q=80" width="753" height="500" lazy>
  <img data-src="https://images.unsplash.com/photo-1510414842594-a61c69b5ae57?w=753&q=80" width="753" height="500" lazy>
  <img data-src="https://images.unsplash.com/photo-1471922694854-ff1b63b20054?w=753&q=80" width="753" height="500" lazy>
  <div data-src="https://images.unsplash.com/photo-1473186578172-c141e6798cf4?w=753&q=80" class="lazy-bg" lazy></div>
  <div data-src="https://images.unsplash.com/photo-1487349384428-12b47aca925e?w=753&q=80" class="lazy-bg" lazy></div>
  <div data-src="https://images.unsplash.com/photo-1527437934671-61474b530017?w=753&q=80" class="lazy-bg" lazy></div>
  <div data-src="https://images.unsplash.com/photo-1473496169904-658ba7c44d8a?w=753&q=80" class="lazy-bg" lazy></div>
img {
  display: block;
  max-width: 100%;
  margin: 10px auto;
  border: 0;
}

.lazy-bg {
  width: 753px;
  height: 500px;
  margin: 10px auto;
  background-size: cover;
  background-position: center;
}
// duyệt tất cả tấm ảnh cần lazy-load
const lazyImages = document.querySelectorAll('[lazy]');

// chờ các tấm ảnh này xuất hiện trên màn hình
const lazyImageObserver = new IntersectionObserver((entries, observer) => {
  entries.forEach((entry) => {
    // tấm ảnh này đã xuất hiện trên màn hình
    if (entry.isIntersecting) {
      const lazyImage = entry.target;
      const src = lazyImage.dataset.src;

      lazyImage.tagName.toLowerCase() === 'img'
      // <img>: copy data-src sang src
        ? lazyImage.src = src

      // <div>: copy data-src sang background-image
      : lazyImage.style.backgroundImage = "url(\'" + src + "\')";

      // copy xong rồi thì bỏ attribute lazy đi
      lazyImage.removeAttribute('lazy');

      // job done, không cần observe nó nữa
      observer.unobserve(lazyImage);
    }
  });
});

// Observe từng tấm ảnh và chờ nó xuất hiện trên màn hình
lazyImages.forEach((lazyImage) => {
  lazyImageObserver.observe(lazyImage);
});

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